This invention pertains to the art of rivets, and particularly to aircraft rivets. By way of background, in order to better understand the invention, in the early days of metal aircraft, aluminum sheets and other parts were riveted together with aluminum rivets made of relatively ductile alloys of aluminum. This arrangement operated very well for many years so long as the rivets had adequate shear strength to develop a balance between shear of the rivet and bearing strength of the parts being fastened.
As demands on aircraft increased stronger alloys of aluminum were used for the sheets, but it was not possible to increase the allowable shear strength of the rivets. Rivets made of these stronger aluminum alloys could not be driven properly; i.e., the rivets often cracked in attempting to expand sufficiently for the desired upset diameter and to properly fill the hole. Relatively larger rivets were required in order to develop the strength of the newer aluminum alloys, and such larger rivets require greater edge distances and wider mounting flanges, thereby disadvantageously increasing total aircraft weight.
To solve this fastener problem, the industry first attempted the use of both steel and stronger aluminum alloy rivets which, however, were not designed to be upset in the conventional rivet manner, but rather were designed to have some sort of collar which was often swaged onto the fastener. These designs, sold under various trademarks, did function and were stronger and did have the necessary strength compared to upsettable aluminum rivets, but they suffered from the severe disadvantages that they were substantially heavier and substantially more costly than true rivets. These fasteners were most often installed in clearance type holes, and thus did not expand and did not fill the hole. The disadvantages of space between fastener and hole was not even recognized as a potential problem at that state of the art, as it is today and as is wellknown to those skilled in riveting.
Next came interference type fasteners of various grades of steel or titanium. A closely allied development was a mating taper between the hole and the shank of the fastener. These fasteners, both tapered and plain, were forced into their holes to solve the previous problems of lack of hole fill. These fasteners, again sold under various different trademarks, depend upon a threaded end and a mating threaded part to close up on the work. Such systems suffer from the disadvantages that they require holes drilled to close tolerances and are very intolerant of any out-of-roundness. The invention, on the other hand, can even tolerate a slightly oblong hole. These threaded type fasteners were heavier yet and even more expensive than the swaged collar types, but they did solve the problem of hole fill.
Then the aircraft industry went to large scale use of titanium in the airframe and titanium rivets which were otherwise similar to solid aluminum rivets. Titanium has sufficient shear strength for aircraft, and since the result was a hard metal rivet in hard metal structure, the resulting assemblages were satisfactory; the strength of the threaded types and the swaged collar types of fasteners was at least equaled at this stage. However, there are problems in the use of solid titanium rivets, which are overcome by invention cavity rivets made of the same titanium alloys. First of all, it is difficult to upset such a solid rivet, and large riveting guns or squeezers are required. Such large tools in turn produce the disadvantages of increased possibilities of damaging adjoining structure, worker fatigue to the point of excluding female riveters (a common practice in the aircraft industry), and producing generally slower, more expensive work. Another problem is that such solid titanium rivets could not be used in aluminum structure in that they would deform the aluminum upon being driven and expanded. This sheet deformation and corresponding internal stress problem using solid titanium rivets in aluminum structure is so great that it is simply not done.
Various modified or compromise approaches have been developed and are currently marketed. The previous logic has simply been reapplied to a titanium rivet; i.e., various kinds of collars or washers are provided around the rivet on the back sheet side to prevent excessive rivet expansion. These systems suffer from new problems including the fact that they require special tools; i.e., they preclude the use of the extensively developed art of conventional riveting thereby wasting that enormous pool of well developed tools and skills. Such systems suffer from a corresponding excessive high price.
Another approach is the bi-metallic rivet. This fastener has a tip, usually welded to the main titanium shank, formed of a more easily upsettable material, such as a relatively soft titanium alloy, a nickel-cobalt alloy, or the like. Bi-metal rivets suffer from a number of disadvantages which are overcome by the invention. The dissimilar metals can cause a hydrogen build-up at the weld interface which can cause the rivet to fail. Many aircraft designers will not risk a failure of the weld joint. The location of the weld joint with respect to the shear plane between the sheets is not inspectable, thereby resulting in the possibility that the soft metal part of the rivet will be located at the structure shear plane. In the invention cavity rivet, the location of the bottom of the cavity with respect to the outer surface of the bottom sheet is inspectable by insertion of appropriate measuring tools into the cavity after the rivet has been upset. In bi-metal rivets it is impossible to locate the weld joint in a driven rivet.
The term "prior art fasteners" as used herein shall be understood to include all of the above fasteners which are not true rivets. That is, the invention cavity rivet is closely related to ordinary rivets in that it is driveable using present technology and in that it is a one-piece single material device. The above term includes all such fasteners which are not driveable, or which involve at least a two-part structure. In summary, all such prior fasteners, as compared to all driveable rivets including the invention cavity rivet, suffer from many disadvantages, the most important ones of which are their relatively heavier weight, relatively higher initial cost, weld failure potential, and relatively higher cost of installation.
In regard to relative costs, as a rough estimate for the purpose of indicating orders of magnitude rather than for exact comparison, assume that a rivet of a certain size in solid titanium would cost about 10 cents, the same rivet formed with a cavity in accordance with the invention would cost about 12 cents. The special solid titanium rivet with its mating collar would cost about 26 cents. An equivalent plain shank threaded fastener would cost about 40 cents, and a tapered interference type would cost about $1.00 and about $4.00 installed in its special accurate tapered hole. An equivalent bi-metal rivet would cost about 16 cents. The same size (although not structurally equivalent) aluminum rivet with the invention cavity would cost about 2 cents and the same wholly conventional solid aluminum rivet would cost about 1/4 of one cent.
A problem will all prior rivets is the possibility of the rivet being overdriven; i.e., excess energy being applied to the rivet in upsetting it. Overdriving can result in the rivet cracking or clinching, or can cause the shank to overswell which can distort the sheets.
An important advantage of the invention resides in its characteristic of change in rate of deflection against upsetting load or force applied. The standard curves are smooth, there are no sharp points of change. The cavity rivet curve, unexpectedly, produces a curve which resembles a step. The second break in the curve occurs at an energy and deflection which produces an acceptable driven rivet and assembled structure, and thereby yields important advantages. Upon reaching this point on the curve, the operator will experience strong resistance to further driving force, and because the curve thereafter rises, even if the operator ignores this "signal", the rivet will tend not to deflect further, and only a concentrated intentional effort to spoil the rivet will produce an unsatisfactory joint. This characteristic is not known to exist in any other kind of rivet.
The invention rivets, as mentioned above, have the important advantage of weight saving over comparable fasteners. Any weight reduction in aircraft is highly desired. Cavity rivets have a weight of 37 percent to 43 percent of the equivalent steel rivets, 30 percent to 60 percent of equivalent prior art fasteners, depending upon size and type, and have slight weight advantage over equivalent bi-metal rivets.
The invention is best applied to a particular installation of a rivet and the sheets or work which the rivet holds together. The length of the cavity in the rivet especially is determined by the thickness and strength of the bottom sheet, i.e., the sheet adjacent the upset end of the rivet. The diameter of the cavity, and the resultant wall thickness defined by the cavity diameter with respect to the rivet diameter, will be determined by various parameters in a particular installation. The various considerations and the teaching of the invention in regard to proportioning of parts in an installation of rivet plus work is more fully set forth below.
A critical parameter in riveting generally and especially in the enviroment of the invention wherein relatively hard metal rivets are used in relatively soft metal work, is rivet expansion. This expansion can be great enough to deform the sheets, which is a serious prior art problem. The formed or cavity end of the invention rivet minimizes this expansion in that it allows a more rapid riveting or formation of the free end upset. This resistance to overswelling at the free end yields the advantage that the assembled sheets are subjected to less deformation. This advantage in turn results in improved fatigue strength. Unfortunately, as is known, there is no agreement among the various companies in the aerospace industry as to standard tests for fatigue, and thus it is not possible to show the advantages of the invention rivet in fatigue on an absolute basis. However, using the standard tests of the Assignee of the present invention, it has been demonstrated that, all other conditions being equivalent, the cavity rivet of the invention has improved fatigue strength over a solid rivet of the same size and material.
The invention cavity rivet controls the flow of material in both the free upset end and the shank inside the work, while at the same time improving the fatigue strength of the assemblage, and permitting faster riveting. In aircraft it is common practice for all rivets to have a tensile strength in the upset end greater than the tensile strength of the head end. Designing for this end result is a consideration in specifying the dimensions of a specific cavity in a specific rivet. However, generally, other considerations, described herein, in typical applications will permit easy achievement of this desideratum.
The invention is not to be confused with tubular rivets which are very old in many non-analogous arts, such as holding pieces of leather or layers of fabric or other soft things together, holding wooden handles on kitchen knives, and the like. A tubular rivet is simply a short length of metal tube which is headed in one way or another. In use, the open tubular end is "clenched" by being struck or otherwise collapsed, or it is frequently simply splayed out, as by having a tool with a conically shaped end forcibly inserted into its open end. The material of the tubular rivet must crack and break in this process, which is acceptable for simple jobs as are mentioned above. In other types of closing methods the open tubular end may be splayed out and then curled inwardly in order to give a neater appearance and to hide the sharp, broken edges. Tubular rivets, even if they were formed of "space-age" materials, would not suffice in aircraft for the dual reasons that there would be insufficient metal and hence insufficient strength at the shear plane, and for the second reason that the tubular end must crack in closing on the work. It is a vital requirement that aircraft rivets have no cracks or other discontinuities in the upset end, or anywhere in the rivet for that matter, because even a microscopically small hairline crack can rapidly propogate itself and cause the rivet to fail under the severe conditions of stress and vibration commonly encountered in aircraft.
Thus, in summary, tubular rivets cannot be used in aircraft and are totally unrelated in concept to the very particularly defined cavity rivet of the invention. A tubular rivet is designed not to have any shaft expansion, thus there is no hole fill. This type of fastener is totally inapplicable where any substantial resistance to fatigue failure is demanded.